El Cap is a fine version of the OS, I intend to stick with it indefinitely on both my 2012 Mini and 2015 MBPro. Absolutely no plans to "move further forward" -- I see at least two more years on El Cap for me, possibly more.
Be aware that quite a few folks who "moved up" to High Sierra have had problems with it.
This doesn't mean that YOU will have problems with it.
But... you might.
In that case, "going back" could be problematical if you don't "prepare yourself" before taking the plunge.
What I would recommend (short version):
1. Before trying High Sierra (or even Low Sierra), make a BOOTABLE CLONED BACKUP of your existing El Cap setup on an external drive. CarbonCopyCloner or SuperDuper will do this quickly and easily. I like CCC because it clones the recovery partition as well.
Both CCC and SD are FREE to download and use for 30 days.
If you have a bootable cloned backup, and don't like HS, it will be child's play to "get back, get back, get back to where you once belonged".
If you DON'T have a cloned backup, you can still get back, but it WON'T be "easy".
2. Do the install FROM A USB FLASH DRIVE --- NOT from the Mac hard drive itself (or from "internet recovery").
Things seem to go better this way.
You need an 8gb or 16gb USB flashdrive and the free little app called "Boot Buddy":
Boot Buddy – sqwarq
Takes only a few minutes and a few mouse clicks to create it.